1. Field of the Invention
The present invention relates to a method of controlling electric charge of a toner and, particularly, to a method of controlling electric charge of a toner by selecting a combination of a toner and a carrier by utilizing a contact potential difference between the toner and the carrier.
2. Description of the Prior Art
Accompanying a rapid development in the information technology on a global scale in recent years, copying machines, facsimiles and printers are growing as output units which are indispensable in the information network society. Many of these information equipment are applying an electrophotographic process by utilizing electric charge due to friction of particles. In the industries of manufacturing leading devices such as in the manufacture of semiconductors and liquid crystal displays which are the key devices mounted on the information equipment and affect performance thereof and in the manufacture of medical products, on the other hand, the electrically charged particles adhere to the devices and to the tubular walls to adversely affect the quality of the final products. In any way, at present, it has been urged in various fields of industries to forward basic studies for controlling electrically charging properties of the particles.
The electrophotographic process can be roughly divided into a one-component developing system and a two-component developing system depending upon the system for electrically charging the toner which is a powdery ink used for developing electrostatic latent image. In the one-component developing system, the electric charge is generated by bringing the toner into pressed contact with a blade of a metal or a rubber. In the two-component developing system, the electric charge is generated by stirring and mixing the toner and carrier particles of iron or ferrite. In either system, it is the most important object to control the amount of electric charge of the toner to lie within a predetermined range to obtain a vivid copy image.
At present, however, no method has yet been established to quantitatively evaluate the electrically charging properties of the toner, and a method (blow-off method) has chiefly been employed to measure a specific charge that generates when the toner and the carrier particles are electrically charged by friction.
The above-mentioned conventional method, however, is capable of experimentally finding the amount of electric charge for a particular combination of the toner and the carrier only, but is not capable of estimating the amount of electric charge for a wide range of combinations of the toner and the carrier maintaining precision.
Here, a contact potential difference specific to a substance is drawing wide attention as a property for quantitatively measuring the electric charge of the toner, and studies have been vigorously conducted.
The contact potential difference will now be described. When a substance and another substance come in contact, in general, a potential difference occurs on the interface. The potential difference differs depending upon the kind of the substance and the surface state. The potential difference on the interface is called contact potential difference. If a work function on the surface of a substance A is denoted by .O slashed..sub.A and a work function on the surface of a substance B by .O slashed..sub.B, the contact potential difference V.sub.O is expressed by the formula (6), EQU V.sub.O =-(.O slashed..sub.A -.O slashed..sub.B)/E (6)
where e is an elementary electric charge (1.6.times.10.sup.-19 C).
According to studies reported so far, however, the sample toner is formed in the form of a plate, or correlation to the amount of electric charge (specific charge) is, in many cases, studied by quoting values disclosed in the literatures. Such methods may indicate a tendency of electric charge but never makes it easy to correctly predict the amount of electric charge and never makes it possible to select a combination of a toner and a carrier in relation to the required amount of electric charge.